The present invention generally relates to communications systems and, more particularly, to a television (TV) system.
For Orthogonal Frequency Division Multiplexing (OFDM) style systems, data is transmitted in groups known as OFDM symbols. Each OFDM symbol is made up of some number of individual carriers that may have either the same, or different, modulation on each carrier. Since each OFDM symbol is different, they have different frequency characteristics. One example of such a system is a Digital Video Broadcast Terrestrial (DVB-T) style system such as DVB-T2. For a DVB-T2 system, the fast Fourier transform (FFT) for use in generating each OFDM symbol can have the following sizes: 1K, 2K, 8K, 16K or 32K (where K=1024), also referred to as carrier modes in DVB-T2, e.g., the 8K carrier mode. In addition, for the 8K, 16K or 32K FFT sizes, there can be a normal carrier mode and an extended carrier mode. In the extended carrier mode, additional carriers are added at both ends of the channel spectrum to allow for optimum use of the channel.
In any communications system various types of interference may be present such as inter-symbol interference (ISI), adjacent-channel interference (ACI), etc. In the context of an OFDM style system, ACI is interference caused by extraneous power from a transmitted signal into an adjacent channel (for DVB-T2, channel spacing may, e.g., be 8 MHz). Advantageously, for a DVB-T2 system, the larger the size of the fast Fourier transform (FFT) causes the spectrum of the transmitted signal to roll off more rapidly outside the channel band—thus limiting the effects on an adjacent channel. In addition, there is typically an analog RF (radio frequency) filter before the transmission antenna to further limit the effects of the transmitted signal on an adjacent channel.
Notwithstanding the above, there may still be instances of ACI even in a DVB-T2 system. One known solution is by offsetting channels. For example, if there are three adjacent channels, A, B and C and there was potential interference between, e.g., channels A and B, this could be resolved by offsetting B towards C. However, this may then cause a conflict between channels B and C.